Noise limiter circuit



NOISE LIMITER CIRCUIT Filed May 31, 1941 S I WHIIQ Q t 1 E 91 ii KQ i' s@ bi; .MMM

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Gttomeg Patented Nov. 10, 1942 NOISE LIMITEB CIRCUIT l y Lester T. Fowler, Oaklyn, N. J., assigner to Radio A Corporation of America, a corporation of Dela- Applicaties my si, 1941, serial No. 395,919

' (Ci. 25o-zo) claims.

This inventionv relates to a noise limiter circuit for a radio signaLreceiving system of the type comprising a series connected diode rectiiier in a signal transmission path, and automatic level setting or bias potential control means for establishing the limiter operating level. y

Noise limiter circuits of this type for reducing noise impulses in radio receiving systems and the like are well known, the type to which the invention relates more particularly, being shown for example in the U. S. Patent to Kaar, 2,207,587. In the system disclosed. a series type noise limiter dlode rectier is provided with means for applying a biasing potential thereto from the audio frequency detector load resistor. However, because of the contact potential effect, both diode rectiilers, used in the detector circuit and as -a limiter, remain in a conducting condition in the absence of a carrier wave, and consequently substantially no limiter action is provided in response to noise impulses without the carrier wave.

It is therefore an object of this invention to provide an improved series' diode noise limiter circuit which responds to weak noise impulses and noise impulses in the absence of a carrier wave, thereby to provide effective overall limiting under all conditions of operation.

The present system is particularly useful in preventing noise between can'ier waves in tuning a receiving-system through a predetermined frequency band such as the present broadcast band.

It is also an object oi' this invention to provide an improved series type noise limiter system in 'which `the contact. potential of n limiter diode rectifier is neutralized in the absence of a carrier the invention is pointed out in the appended claims. v

Referring to the drawing, the figure is a schematic circuit diagram of a portion of a radio signal receiving system embodying the invention and including an intermediate frequency ampliiler output stage 5 coupled through a suitable intermediate frequency coupling transformer 6 to a second or audio frequency detector comprising a diode rectier having an anode 1 and a cathode 8. The anode 1 is connected with thehigh potential terminal 9 of the transformer 5 while the cathode l is connected through an output load resistor iii- Il to the low potential output terminal l2 of the transformer t. The resistors Il and Il are provided with an intermediate frequency bypass capacitor i3 thereby completing the usual diode rectifier circuit.

The cathode l is also connected to chassis or ground, as indicated at Il, through a bias resistor I5 provided with anaudio frequency by- .pass capacitor I6. This is for the purpose of providing a biasing potential for an amplifier i control grid I1 common to the cathode 8 in the same envelope with the diode rectier anode 1,

the ampliier output anode being indicated at i8.

wave and compensated or neutralized automaticallywithout interfering with the normal'reception of a carrier wave or signal.

-It is also a further object of this invention to provide a noise limiter circuit which is not only i effective in the absence of a carrier wave but also involves a minimum number of circuit elements,

whereby it is applicablev to commercial receiving systems.

The invention is particularly applicable to diode rectifier circuits involving the second or audio-frequency detector of a superheterodyne or other signal receiving system having the usual output load resistor, anfd the invention will be I described, by way of example, in connection with such a receiving system. Fora further understanding of the invention, attention is now directed to the accompanying drawing, together `with the following description. and the scope of 55 This amplifier represents any suitable audio frequency amplier stage for the detector and is coupled to any suitable utilization circuit through an output circuit indicated at 2l.

The control grid i1 is connected through a lead 2| with a suitable volume control potentiometer 22-23, one side of which is connected to ground or chassis 24 while the high potential terminal is coupled through a coupling capacitor 25 with a tap 2t on the detectoroutput resistor Ili-i I.

Interposed in this connection is a series limiter diode rectifier comprising an anode 21 connected through a lead 2l with the 'terminal .26

a cathode v29 connected through a lead '30 .with a terminal Il to which the coupling capacitor 25 is connected,thereby completing the circuit between the signal utilization means comprising the amplliier stage and the detector output load resistor 'l-I I.

'me cathode 2s of the series umiter diode reetiiler is connected with the terminal l2 of the transformer 6 which is also connected to the outer terminal of the resistor section il of the detector output resistor. through a series regulating resistor 35 shunted by a suitable switch 3B, a potential source represented by a resistor 31 and a filter resistor 38 provided with a bypass capacitor I! to ground or chassis as indicated at 4l.

Since the cathode l is connected to the outer il, audio frequency signals are derived from the resistor section il. with the output circuit represented by thepotentiometer 23 connected substantially in parallel therewith through the coul pling capacitor Il. the series limiter diode rectiiler l 21-29 and the ground connections :L -I4.

In, the `presence'of a carrier wave, the limiter diode rectiiier 2l-'4l is maintained in an operv 'ating'and automatic level setting condition by the c signal current ilow through the other or current supply section Il of the output resistor, current sowing from the terminal :s to' the anode 21, through the rectiiler-tothef--cathodc 20, thence thesourceattheterminall2.

A signal peak or noise pulse exceeding B lire- 4 through the resistors u. u and saone beek 'to for example, causes'the anode 21 oi the diode rectiiier 21-20 to become negative with respect to its. cathode 20', thus interrupting the current and opening the output circuit, thereby providing cut-oi! of the peak or noise pulse. The

percentage modulation at which the noise limiter operates is determined in part by the ratio of the resistor sections il and Il. In the: present example these may be considered to be of the order'v of 100,000 and 150,000 ohms, respectively. The illter resistor 3l may have a value of l megohm with a bypass capacity at Il of '.1 mid.

In the absence o! a carrier wave. the limiter diode rectiiler is rendered non-conducting or open, by reasonoi the voltage existing across the source 31. having a polarity as indicated. with respect to the polarity of the output resistor Il--IL The potential oi this source may be supplied by the; contact potential of a third diode. for the reason .that the resistors i iare in the limiter diode circuit along with the resistor 31, -while the latterresistor' alone provides the full potential drop in circuit g with the control diode.

I claim as my invention:

an output impedance, a limiter diode rectiiler connected with said impedance, means providing f an alternating' current signal output path through said limiter i'roma portion of said imo pedance, means providing a direct current condetermined oereentoee modulation sueno-e 100% l trol path through said limiter from asecond portion of said impedance, and a diode control rectifier connectedin said last named control pathY for neutralizing the contact potentialvoi said limiter. whereby noise pulse limiting is eilective 'in the absence oi a carrier wave.

2. The combination oi a carrier wavesignal conveying circuit, ot a signal detector having an output impedance, a limiter diode rectiiier connected withA said impedance, means providing an alternating current signal output path through said limiter from a portion oi' said impedance, means providingl a direct'curropt condiode rectifier comprising an anode 4l and a cathode Il connected across the resistor Il. This provides an initial negative voltage on the limiter diode anode 21. Therefore weak noise pulses, in the absence of a carrier wave will be suppressed since the contact potential ofthe diode rectiner IB-II is in eil'ect connected in opposition to that of the y limiter diode rectiiler 21-2l. In the present example the contact potential oi the control diode 45-40 is generated across the resistor 31 and this voltage is applied to the limiter diode through the resistors Il, li and IE. The latter resistor may be omitted as by closing the switch 38, in which case the limiter actionvis more-eiective in that when a noise pulse causes the limiter diode 21-21 to become non-conducting, the diode ll-IO is conducting because it is' then subjected to its own contact potential only,

and the low impedance oi' the last-named diode provides a short-circuit effect across the signal output potentiometer 23 from the high side thereoi' at terminal A3| to ground 24' through the capacitor Il.' In the presence of signal current the diode rectiner-l is cut oi! because of the voltage drop .through the resistor 31. vThe control diode is thus of low resistance atone phase of the operation and of inilnite resistance during another phase of the operation, as desired.

for enective control of the limiter action.

Thus it will be seen in the present system, an

eil'ective ln the absence oi' a carier wave. It will benoted that the limiter diode contact potential- 'does not apply a` similar control to the` control troly path through said limiter from a second portion ci said impedance. a diode control rectiiler connected in said last named control path for neutralizing the contact potential Vof said limiter, whereby noise pulse limiting is eiiective inthe absence of a carrier wave, and means in circuit with said rectiilers for limiting the control euect of the limiter diode contact potential on said control diode.

.3. The combination with a carrier wave signal lconveying circuit, of a signal detector having an output impedance, a limiter diode rectiuer connected therewith, means providing an alternating current signal output path through said limiter from a portion oi' said impedance, means providing a direct current control path through said vlimiter from a second portion of said impedance. a diode control rectiner connected in said last control path for neutralizing the contact potential of said limiter. whereby noise pulse limiting is effective in the absence oi a carrier wave, and means including a shunt potential drop producing resistor for said control rectifier for establishing a cut-ot! potential ldrop for said rectifier in the presence ofv a received carrier wave.

4. The combination with a signal detector having a signal output load impedance, of a limiter diode rectiiier, a signal output circuit coupled to -sald load resistor through said limiter diode last named rectiiler including .a illter network and at least a portion oi said output load resistor, a control diode rectiflerconnected in said l last named circuit and polarized to neutralize the contact potential ot said limiter diode rectiiler in the absence of a carrier wave.

5. The combination with a carrier wave signal conveying circuit, of a signal detector having an output impedance, a limiter diode rectifier con- -t nected with said. impedance, means providing an f alternating current signal output path through 'said limiter from a portion oi said impedance,

means providing a direct current control path through' said limiter from a second portion oi' said impedance, and means in said last control f path including a second'rectiiler for neutralizing the Acontact potential of said limiter whereby noise pulse limiting'is eiiective inthe absence-ot.

a carrier wave, and means for cutting oi'f the control rectiiier in the presence of signals..

6. The combination with a signal detector, of a signal input circuit therefor, an output load impedance for said detector having a terminal connected to ground and having a tap thereon, an

audio frequency output circuit connected between said tap and ground, a series limiter diode rectifler in said last named connection between said tap and the high potential side oi said output circuit, a circuit connection between a second terminal of said output impedance and the output side of the limiter diode rectiiler to provide automatic level setting therefor, said connection comprising a. pair of series connected resistors. an audio frequency bypass capacitor connected between the junction of said last named resistors and ground, and a control diode rectifier connected between said junction and the high potential side of the audio frequency circuit and limiter said limiter rectiiler from said output load resistor including a circuit connection providing a direct current path from said load resistor through said limiter rectifier, a control rectiiler connected serially in said last named circuit to provide contact potential in opposition' to the contact potential of the limiter rectifier, a resistor connected in shunt with said control rectiiier, and a lter connected between said last named rectier and the diode output resistor, whereby in the absence of a carrier wave, low amplitude noise pulses are cut oi! from said audio frequency output circuit.

8. In a signal detector having an outputimpedance, a signal output connection including a series diode limiter for deriving output signals from a portion of said impedance, a control circuit for said limiter including a second portion of said output impedance, a control diode rectier having a polarity such that in the absence of a carrier wave signal the contact potential of the limiter rectier is neutralized by the contact potentiai of said control rectifier, means in circuit between said last named rectiilers for limiting the control eiIect of the limiterv diode contact potential on said control diode, and means responsive to rectified signal current for cutting od said control diode rectiiier.

LESTER T.- 

